Easily done to achieve a seamless, fail-over for network traffic. Yes, it is load balancing.
I've actually found your question because I'm looking to find info on using multiple adsl line for gaming
I've used PFSENSE.ORG before for a client of mine. It is a free linux distro that sits on a dedicated computer with multiple Network cards. I had 3 on there. Two ADSL lines into the customers property and then one network card back out again.out, to distribute the combined connectionss around his property. If you have an old computer lying around, then a couple of extra network cards would set you back a few pounds/dollars/yen/milk-bottle-tops
Pulling FTP and using speedtest sites did see his download speeds double.
However, for gaming, the server would see your two IP addresses as two separate connections which is why I am unsure how it would cope. It would be no problem at all to force all traffic for a certain port out of one connection only though, and then a few clicks inside a web interface to tell those ports to go on another connection should one go down.